Deep X-ray lithography involves a substrate which is covered by thick photoresist, typically several hundred microns in thickness, which is exposed through a mask by X-rays. X-ray photons are much more energetic than optical photons, which makes complete exposure of thick photoresist films feasible and practical. Furthermore, since X-ray photons are short wavelength particles, diffraction effects which typically limit device dimensions to two or three wavelengths of the exposing radiation are absent for mask dimensions above 0.1 micron. If one adds to this the fact that X-ray photons are absorbed by atomic processes, standing wave problems, which typically limit exposure of thick photoresist by optical means, become a non-issue for X-ray exposures. The use of a synchrotron for the X-ray source yields high flux densities, several watts per square centimeter, combined with excellent collimation to produce thick photoresist exposures with minimal horizontal run-out. Locally exposed patterns should therefore produce vertical photoresist walls if a developing system with very high selectivity between exposed and unexposed photoresist is available. This requirement is satisfied by polymethylmethacrylate (PMMA) as the X-ray photoresist, and an aqueous developing system. See, e.g., H. Guckel, et al., "Deep X-ray and UV Lithographies for Micromechanics", Technical Digest, Solid State Sensor and Actuator Workshop, Hilton Head, S.C., Jun. 4-7, 1990, pp. 118-122.
As the thickness of the photoresist is increased, e.g., beyond 500 .mu.m, difficulties have been encountered when relatively soft (lower photon energy) X-ray sources are used for photoresist exposure. The wavelengths of soft X-rays are long enough that diffraction effects can lead to penumbral blurring of the X-ray mask shadow image. Difficulties can also arise from interface phenomena caused by the non-negligible reflectivity of common photoresist materials with respect to soft X-rays. It has also been found that for resist sheets thicker than about 500 .mu.m, soft X-rays cannot penetrate sufficiently to the bottom of the resist layer to guarantee development of the bottom regions of the resist without such a large exposure that the dose at the upper regions of the resist would become so high as to cause destructive overexposure. Such overexposure can cause distortion or fracture of the resist layer, or even destruction of the fragile soft X-ray mask which is typically positioned within a few microns of the resist layer. However, these problems can be avoided by exposing thick photoresist layers using hard (high energy) X-rays. Hard X-rays may be obtained by spectral shaping the X-ray beam from a source, such as a synchrotron, having a significant spectral content of high energy X-rays to substantially eliminate lower energy photons which have a short absorption length in the photoresist, and which would tend to be substantially absorbed near the top surface of the photoresist. Spectral shaping for hard X-rays may be accomplished by spectral filtering using, e.g., aluminum and/or silicon filters. The penetrating ability of hard X-rays allows photoresist layers up to several centimeters thick to be exposed, allows multiple stacked photoresist layers and substrates to be exposed simultaneously, and allows the X-ray mask to be formed on robust substrates, such as relatively thick and wide single crystal silicon wafers, on which an X-ray absorber such as gold is deposited in a pattern.
Deep X-ray lithography may be combined with electroplating to form high aspect ratio structures. This requires that the substrate be furnished with a suitable plating base prior to photoresist application. Typically, this involves a sputtered film of adhesive metal such as chromium or titanium, which is followed by a thin film of a metal which is to be plated. Exposure through a suitable mask and development are followed by electroplating. This results, after clean-up, in fully attached metal structures with very high aspect ratios. Such structures were reported by W. Ehrfeld and co-workers at the Institute for Nuclear Physics at the University of Karlsruhe in West Germany. Ehrfeld termed the process "LIGA" based on the first letters of the German words for lithography and electroplating. A general review of the LIGA process is given in the article by W. Ehrfeld, et al., "LIGA Process: Sensor Construction Techniques Via X-ray Lithography", Technical Digest, IEEE Solid State Sensor and Actuator Workshop, 1988, pp. 1-4.
The addition of a sacrificial layer to the LIGA process facilitates the fabrication of fully attached, partially attached, or completely free metal structures. Because device thicknesses are typically larger than 10 microns and smaller than 300 microns, free standing structures will not distort geometrically if reasonable strain control for the plated film is achieved. This fact makes assembly in micromechanics possible, and thereby leads to nearly arbitrary three-dimensional structures. See H. Guckel, et al., "Fabrication of Assembled Micromechanical Components Via Deep X-ray Lithography", Proceedings of IEEE Micro Electro Mechanical Systems, Jan. 30-Feb. 2, 1991, pp. 74-79.
It is possible to extend the LIGA process, with or without a sacrificial layer, for the formation of multi-layer micromechanical structures. This is achieved by performing several X-ray exposures of multiple photoresist layers, with electroplating of additional layers of metal after each exposure. In such a procedure, metal microstructures may be electroplated both onto the substrate plating base and onto the top surface of previously deposited microstructure layers. Replanarization of previously deposited microstructure layers, to achieve a substantially flat uniform surface, is typically required before electroplating subsequent metal microstructure layers thereon. Various machining, lapping, and polishing procedures may be used for replanarization.
A key to successful fabrication of multi-layer micromechanical device structures is the accurate alignment of subsequently deposited microstructure layers with previously deposited ones. For example, if a gear-shaft-gear combination is to be fabricated, with each component of the combination formed on a different microstructure layer, only a small eccentricity between the axes of the gears and shaft, and thus only a small alignment error between the microstructure layers, can be tolerated. Hence, a procedure to achieve high alignment accuracy between layers must be available in order to extend LIGA for the fabrication of multi-layer micromechanical devices.
Alignment procedures similarly play an important role in the related field of microelectronics fabrication. Microelectronics manufacturing uses alignment procedures for the fabrication of multi-level integrated circuits. Alignment procedures used in Very Large Scale Integration (VLSI) processing for microelectronics, for example, are usually of an optical nature. In the VLSI process, each level is formed to contain reference marks at a certain location. These marks are used for optically aligning a subsequent mask to a previous level. A microscope is focused on the alignment marks on both the previous work level and the subsequent level mask. The mask is then positioned such that the marks on the work level and the mask are optically aligned with each other, thus aligning the subsequent mask pattern with the previous level microelectronic pattern. During optical exposure through the subsequent mask, new alignment marks are transferred to the current work level. Using this method, the several layers which make up an integrated circuit are aligned with each other.
Since visible light is used for the optical alignment procedure, the procedure is limited to alignment gaps between the substrate and the mask which lie in the range of the depth of focus of the alignment microscope. In VLSI processing, the alignment gap between the device substrate and the mask is on the order of a few micrometers. However, in LIGA processes for the fabrication of relatively thick metal microstructures, the mask and the device substrate may typically be more than 100 micrometers apart. For the magnification required to optically align the LIGA substrate to a mask with sub-micron tolerance, the depth of focus is much less than 100 micrometers. Moreover, X-ray masks used in LIGA processing are typically formed from a silicon wafer (for hard X-rays) or a silicon wafer which includes a silicon nitride membrane (for soft X-rays) upon which a thin metal plating base layer is deposited for subsequent formation of the mask pattern by plating of an X-ray blocking material, such as gold, onto the plating base layer. The X-ray masks used in LIGA processing in most cases are, therefore, not optically transparent. For these reasons, the optical alignment procedure used in VLSI processing cannot easily be applied to the fabrication of multilayer micromechanical devices by the LIGA process.
An alignment procedure that may be used for fabrication of multi-layer microstructures by the LIGA process is suggested in U.S. Pat. No. 5,378,583, to Guckel. et al., entitled "Formation of Microstructures Using a Preformed Photoresist Sheet". This patent describes the use of thick preformed sheets of photoresist for the fabrication of photoresist and metal microstructures. Multiple photoresist sheets may be exposed to X-rays in various patterns, and then adhered together into a laminate. The laminate may then be adhered to a substrate and used for the fabrication of metal microstructures. Alignment of the various photoresist layers, such that microstructure regions formed therein by exposure to X-rays are aligned with each other, may be achieved by creating mechanical alignment structures during exposure of each photoresist layer, and then using these alignment structures to obtain mechanical registration between a previous layer and a subsequent layer. Exemplary alignment structures may consist of relatively large holes formed in each photoresist layer on opposite sides of the patterned portion thereof. The holes may be formed as part of the X-ray exposure and developing process for patterning the photoresist layer, so that the microstructure pattern and the alignment holes are formed simultaneously for each layer, and so that the relative position of the alignment holes with respect to the microstructure pattern is precisely controlled. The alignment holes are designed to accept pegs, which may be formed of a photoresist material or metal. Alignment between photoresist layers is achieved by assembling mounting holes in a subsequent layer onto the pegs placed in mounting holes from the previous layer, and gluing the subsequent photoresist layer to the underlying layer. Self-alignment is obtained because the alignment holes are exposed at the same time as the desired pattern in each layer, and therefore the alignment tolerance is governed by assembly tolerances. After the desired number of photoresist layers have been built up into a laminate, metal may be electroplated into the microstructure pattern regions in the photoresist laminate, if desired, to effectively form a multi-layer metal microstructure.